Can I leave when I graduate or would I have to wait until I turn 18 in September? Your date of graduation is totally irrelevant. While in a few states the law says you are emancipated on your 18th birthday or the day you graduate, whichever is LATER, in NO state does graduation BEFORE the age of 18 emancipate you. You can leave on your 18th birthday or the day your parents give you permission, whichever is earlier. It matters not whether you have graduated or not.
And if I leave in June, when I graduate, before I turn 18, could they call the police on me? Yes.
But if they do call the police, wouldn't me leaving count as beneficial to my education since I'm attending college in California, and I would need to arrive there early to prepare for the upcoming semester? (Fall semester starts in August, if I have to wait until September so I'd be 18, I'd miss registration for Fall semester) Totally irrelevant. You leave when your parents give you permission to leave, or when you are 18. What is and is not "beneficial to your education", in your opinion, does not make the law go away.
Also, if I am able to leave in June, can I just leave without leaving any way for them to contact me? Or would they still count as my legal guardian since I am still 17 and not 18 yet? They are your legal guardians until your 18th birthday and not one minute before. If you leave without permission, whether or not you have left a means of contact, that is called, running away from home, and you can get your little behind hauled right back home.
Once and for all, graduating is NOT relevant to your becoming a legal adult, having the right to leave home, or any other issues you may try to raise. As long as you are 17 years, 364 days old or younger, your parents have 100% control over where you live.

